Efficiency determination for a selective-catalytic-reduction catalyst

ABSTRACT

A method of assessing overall efficiency of a selective-catalytic-reduction catalyst includes monitoring instantaneous efficiency of the catalyst. The method also includes determining the overall efficiency by summing instantaneous efficiency values weighted by a first set of coefficients if the most recent instantaneous efficiency value is above an instantaneous efficiency threshold. The method additionally includes determining the overall efficiency by summing instantaneous efficiency values weighted by a second set of coefficients if the most recent instantaneous efficiency value is equal to or below the instantaneous efficiency threshold. Furthermore, the method includes determining whether the overall efficiency has dropped below an overall efficiency threshold and reporting when the overall efficiency has dropped below the overall efficiency threshold. The second set of coefficients weighs the most recent instantaneous efficiency value more heavily than preceding instantaneous efficiency values as compared with the first set of coefficients. A system and a vehicle are also disclosed.

TECHNICAL FIELD

The present invention is drawn to a system and a method for determiningefficiency of a selective-catalytic-reduction catalyst.

BACKGROUND

Selective catalytic reduction (SCR) is a chemical process used forconverting oxides of nitrogen (NO_(x)) with the aid of a catalyst intodiatomic nitrogen (N₂) and water (H₂O). In modern motor vehicles, SCR isfrequently used as part of an exhaust emissions control system of aninternal combustion engine to reduce the subject engine's release ofpost-combustion NO_(x) into the atmosphere. Gasoline, as well as diesel,engine emissions may be treated with SCR.

In a vehicle equipped with SCR, a reductant, which may be an aqueoussolution of urea, is typically injected into the engine's exhauststream. An SCR equipped vehicle typically carries its reductant onboardin a specially designed reservoir. The reductant is dosed into theengine's exhaust stream during engine operation in proportion toconsumption of the engine's fuel. Once in the exhaust stream, thereductant is absorbed onto the system's SCR catalyst where theconversion of NO_(x) takes place.

SUMMARY

A method of assessing overall operating efficiency of aselective-catalytic-reduction (SCR) catalyst includes monitoringinstantaneous efficiency of the SCR catalyst. The method also includesdetermining the overall operating efficiency of the SCR catalyst bysumming values of the instantaneous efficiency weighted by a first setof coefficients if the most recent value of the instantaneous efficiencyof the SCR catalyst is above an instantaneous efficiency threshold. Themethod additionally includes determining the overall operatingefficiency of the SCR catalyst by summing values of the instantaneousefficiency weighted by a second set of coefficients if the most recentvalue of the instantaneous efficiency of the SCR catalyst is equal to orbelow the instantaneous efficiency threshold. Furthermore, the methodincludes determining whether the overall operating efficiency of the SCRcatalyst has dropped below an overall operating efficiency threshold andreporting when the overall operating efficiency of the SCR catalyst hasdropped below the overall operating efficiency threshold. According tothe method, the second set of coefficients weighs the most recentinstantaneous efficiency value more heavily than preceding instantaneousefficiency values as compared with the first set of coefficients.

The act of determining the overall operating efficiency of the SCRcatalyst by summing values of the instantaneous efficiency weighted bythe second set of coefficients may be accomplished for a predeterminednumber of values of the instantaneous efficiency.

The act of determining the overall operating efficiency of the SCRcatalyst may include employing an exponentially-weighted moving average(EWMA) filter. Accordingly, the EWMA filter may be employed to decreasethe effect of more recent instantaneous efficiency values on the overalloperating efficiency determination.

The SCR catalyst may be employed for reducing oxides of nitrogen(NO_(x)) emissions from an internal combustion engine. The engine may bea diesel type and the SCR catalyst may employ a diesel-exhaust-fluid(DEF) as a reductant supplied from a reservoir for reducing NO_(x)emissions. Furthermore, the predetermined number of values of theinstantaneous efficiency may be collected during a single key cycle ofthe engine.

The overall operating efficiency of the SCR catalyst being below theoverall operating efficiency threshold may be indicative of the SCRcatalyst having failed.

Monitoring the instantaneous efficiency of the SCR catalyst, determiningthe overall operating efficiency of the SCR catalyst, determiningwhether the overall operating efficiency of the SCR catalyst has droppedbelow the overall operating efficiency threshold, and reporting when theoverall operating efficiency of the SCR catalyst has dropped below theoverall operating efficiency threshold may be accomplished by acontroller.

According to the method, the act of reporting when the overall operatingefficiency of the SCR catalyst has dropped below the overall operatingefficiency threshold may be accomplished by generating an alert to anoperator of the engine.

A system for assessing overall operating efficiency of an SCR catalystand a vehicle employing such a system are also provided.

The above features and advantages and other features and advantages ofthe present invention are readily apparent from the following detaileddescription of the best modes for carrying out the invention when takenin connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of vehicle with an engine connectedto an exhaust system having a selective-catalytic-reduction (SCR)catalyst; and

FIG. 2 is a flow diagram of a method for assessing overall operatingefficiency of the SCR catalyst of FIG. 1.

DETAILED DESCRIPTION

Selective catalytic reduction (SCR) is a means of converting oxides ofnitrogen, also referred to as NO_(x) with the aid of a catalyst intodiatomic nitrogen, N₂, and water, H₂O. A gaseous reductant, typicallyanhydrous ammonia, aqueous ammonia or urea, is added to a stream ofexhaust gas and is absorbed onto the catalyst. SCR is frequentlyemployed to reduce NO_(x) emissions in the exhaust of internalcombustion engines used to power motor vehicles. Exhaust emissions ofboth gasoline and diesel engines may be improved by SCR. A general term“diesel-exhaust-fluid” or DEF is used to describe a reductant that isemployed by SCR in diesel engines.

Referring to the drawings, wherein like reference numbers refer to likecomponents throughout the several views, FIG. 1 schematically depicts amotor vehicle 10. The vehicle 10 includes an internal combustion engine12 configured to propel the vehicle via driven wheels 14. Although theengine 12 is a compression-ignition or diesel type as shown anddescribed herein, the engine may also be configured as a spark-ignitionor gasoline type. Internal combustion in the diesel engine 12 occurswhen a specific amount of ambient air flow 16 is mixed with a meteredamount of fuel 18 supplied from a fuel tank 20 and the resultantair-fuel mixture is compressed inside the engine's cylinders (notshown).

The vehicle 10 also includes a system 22 configured to assess overalloperating efficiency of a selective-catalytic-reduction (SCR) catalystthat is employed for treating noxious emissions contained in an exhaustgas stream 24 of the engine 12. The exhaust gas stream 24 is emittedfrom the engine 12 as a by-product of combustion, and is removed to theambient through an exhaust system 26. The exhaust system 26 includes aseries of exhaust after-treatment devices, shown as a diesel oxidationcatalyst 30, an SCR catalyst 32, and a diesel particulate filter 34.Accordingly, the exhaust system 26 includes a passage for directing theexhaust gas stream 24 from the engine 12 through the series of exhaustafter-treatment devices. The shown series of exhaust after-treatmentdevices 30, 32, and 34 is employed to reduce various exhaust emissionsof engine 12. In particular, the diesel oxidation catalyst 30 is adaptedto receive exhaust gas stream 24 from the engine 12 to oxidize and burnhydrocarbon emissions present in the exhaust gas. Following the dieseloxidation catalyst 30, the exhaust gas stream 24 is routed to the SCRcatalyst 32, which is employed to reduce the emission of NO_(R).

As shown in FIG. 1, the system 22 includes a reservoir 36 configured tosupply a metered amount of a DEF 38 into the exhaust gas stream 24upstream of the SCR catalyst 32. Accordingly, the DEF 38 accesses theSCR catalyst 32 as the exhaust gas stream 24 flows through SCR catalyst.An inner surface of the SCR catalyst 32 includes a wash coat 40. Thewash coat 40 serves to attract the DEF 38 in order to deposit the DEFwithin the SCR catalyst 32 such that the DEF may interact with theexhaust gas stream 24 and generate a chemical reaction to reduce NOemissions from the engine 12. After the exhaust gas stream 24 exits theSCR catalyst 32, but before it is allowed to pass to the atmosphere, thegas stream is routed through the diesel particulate filter 34 where thesooty particulate matter emitted from the engine 12 is collected anddisposed through a regeneration process. Although, as shown, the SCRcatalyst 32 is positioned upstream of the diesel particulate filter 30,the SCR catalyst may also be positioned downstream of the dieselparticulate filter without affecting the effectiveness of the exhaustafter-treatment devices 30, 32, and 34 in the after-treatment of theexhaust gas stream 24.

The SCR catalyst 32 is characterized by operating efficiency that isdetermined by the effectiveness of the catalyst in reducing NO_(x)emissions from the engine 12. The structural integrity of the wash coat40 is a major factor responsible for the operating efficiency of the SCRcatalyst 32. The wash coat 40 may become degraded as a result of thermalstress generated by the exhaust gas stream 24 such that the wash coatbecomes incapable of retaining the DEF 38 on the inner surface of theSCR catalyst 32. Accordingly, when the wash coat 40 becomes degraded,the chemical reaction necessary to reduce NO_(x) emissions from theengine 12 cannot be sustained and, as a result, the operating efficiencyof the SCR catalyst 32 also suffers.

The system 22 also includes a controller 42. The controller 42 may be astand-alone unit, or be part of an electronic controller that regulatesthe operation of engine 12. The controller 42 is configured to monitorinstantaneous efficiency of the SCR catalyst 32 by receiving from aprobe 44 a signal indicative of the amount of NO_(x) emissions remainingin the exhaust gas stream 24 after the exhaust gas stream has passedthrough the SCR catalyst. The controller 42 is programmed to receive thesignal from the probe 44 at a predetermined rate that would besufficient to detect any significant changes in the operating efficiencyof the SCR catalyst 32 during operation of the engine 12. The system 22may also include an upstream probe (not shown). In such a case, a signalfrom the upstream probe would be compared by the controller 42 to thesignal from the probe 44 for detecting changes in the operatingefficiency of the SCR catalyst 32.

The controller 42 is also configured to determine an overall operatingefficiency 43 of the SCR catalyst 32 by summing values of theinstantaneous efficiency communicated by the probe 44. Prior to beingsummed, each of the values of the instantaneous efficiency is weightedby a specific coefficient that is part of a predetermined set ofcoefficients. The instantaneous efficiency may also be determined over aspecific sample size that can be defined by length of time oraccumulated mass of NO_(x) and then integrated to arrive at a specificvalue. Accordingly, instantaneous efficiency may be an integrated sum ofefficiency values taken over an evaluation window. The particular set ofcoefficients to be used in any instance is dependent on an instantaneousefficiency threshold 46 that is predetermined during validation andtesting of the SCR catalyst 32 behind the engine 12 and programmed intothe controller 42.

The instantaneous efficiency threshold 46 is indicative of a boundarydrawn empirically to delineate “normal” or generally effective operationof the SCR catalyst 32 from “abnormal” or degraded operation of the SCRcatalyst at any particular instance. Accordingly, two situations aregenerally contemplated—where the instantaneous efficiency of the SCRcatalyst 32 is above the instantaneous efficiency threshold 46 and wherethe instantaneous efficiency is equal to or below the instantaneousefficiency threshold. Furthermore, two separate courses of action inresponse to these two situations are programmed into the controller 42.

In the first situation, where the most recent detected value of theinstantaneous efficiency of the SCR catalyst 32 is above theinstantaneous efficiency threshold 46, a first set of coefficients 48 isused to weigh each value of the instantaneous efficiency. In the secondsituation, where the most recent detected value of the instantaneousefficiency of the SCR catalyst 32 is at or below the instantaneousefficiency threshold 46, a second set of coefficients 50 is used toweigh each value of the instantaneous efficiency. The second set ofcoefficients 50 weighs the most recent instantaneous efficiency valuemore heavily than preceding instantaneous efficiency values as comparedwith the first set of coefficients 48.

The controller 42 is also configured to determine whether the overalloperating efficiency 43 of the SCR catalyst 32 has dropped below anoverall operating efficiency threshold 52. The condition when theoverall operating efficiency 43 of the SCR catalyst 32 has dropped belowthe overall operating efficiency threshold 52 may be identified as afailure of the SCR catalyst that necessitates the catalyst's servicingor replacement. The overall operating efficiency threshold 52 may beestablished empirically based on the minimum required conversion ofNO_(x) by the SCR catalyst 32 during various maneuvers of the vehicle 10and the corresponding operation of the engine 12, as defined by alegislatively mandated emissions test.

The first and second sets of coefficients 48, 50 may be used as part ofthe determination of the overall operating efficiency 43 of the SCRcatalyst 32 in an exponentially-weighted moving average (EWMA) filterprogrammed into the controller 42. The EWMA filter is a type of aninfinite response filter which applies to discrete data points or valuesweighting coefficients that decrease exponentially. The weighting foreach older data point decreases exponentially, never reaching zero.Therefore, the EWMA filter operates to decrease the effect of morerecent instantaneous efficiency values on the determination of theoverall operating efficiency 43 in order to preclude a prematureassessment that the SCR catalyst 32 has failed. Such a decrease of theeffect of more recent instantaneous efficiency values on thedetermination of the overall operating efficiency 43 is beneficial inaddressing common operating conditions of the vehicle 10 that may resultin significantly increased variability in SCR efficiency determination.Common operating conditions of the vehicle 10 that may result in suchincreased variability in SCR efficiency determination may include, forexample, situations when the vehicle is ascending an extended steepgrade or is pulling a heavy load. Non-steady state operation of the SCRitself may also influence increased variability in SCR efficiencydetermination.

As described above, the second set of coefficients 50 weighs the mostrecent instantaneous efficiency value more heavily than precedinginstantaneous efficiency values as compared with the first set ofcoefficients 48. The second set of coefficients 50 is therefore used toaccelerate a determination of the overall operating efficiency 43 of theSCR catalyst 32. For example, the first set of coefficients 48 may weighthe most recent instantaneous efficiency value as 20% of all theinstantaneous efficiency values being summed in calculating the overalloperating efficiency 43 of the SCR catalyst 32. In comparison, thesecond set of coefficients 50 may weigh the most recent instantaneousefficiency value as 60% of all the instantaneous efficiency values beingsummed in calculating the overall operating efficiency 43 of the SCRcatalyst 32.

As noted above, the accelerated determination of the overall operatingefficiency 43 of the SCR catalyst 32 is initiated by the controller 42once the instantaneous efficiency of the catalyst has dropped to theinstantaneous efficiency threshold 46. Accordingly, such accelerateddetermination of the overall operating efficiency 43 of the SCR catalyst32 by using the second set of coefficients 50 permits a more rapiddetermination of whether the SCR catalyst has failed. Additionally, thespecific weighting of the second set of coefficients 50 may be selectedsuch that the assessment of whether the SCR catalyst 32 has failed maybe reached within a predetermined number of instantaneous efficiencydata points or values collected during a single key cycle of the engine12. The actual number of instantaneous efficiency data points collectedduring a single key cycle of the engine 12 for reaching the assessmentmay be dependent on a legislative enactment, such as the United StatesEnvironmental Protection Agency (US EPA) OBDII, requiring the specificnumber.

The controller 42 is additionally configured to report when the overalloperating efficiency 43 of the SCR catalyst 32 has dropped below theoverall operating efficiency threshold 52. To achieve such reporting,the controller 42 may generate a sensory signal 54 indicative of theoverall operating efficiency 43 of the SCR catalyst 32 being below theoverall operating efficiency threshold 52. Such a sensory signal 54 maybe displayed on an instrument panel 56 of the vehicle 10 as a visualalert in order to notify the vehicle's operator that servicing orreplacement of the SCR catalyst 32 may be required.

FIG. 2 depicts a method 60 of assessing operating efficiency of the SCRcatalyst 32, as described with respect to FIG. 1. Accordingly, themethod commences in frame 62, where it includes monitoring instantaneousefficiency of the SCR catalyst 32. From frame 62, the method advances toframe 64, where the method determines whether the most recent value ofthe instantaneous efficiency of the SCR catalyst is above theinstantaneous efficiency threshold 46, or is at or below theinstantaneous efficiency threshold. If the most recent value of theinstantaneous efficiency of the SCR catalyst is above the instantaneousefficiency threshold 46, the method proceeds to frame 66. In frame 66,the method includes determining the overall operating efficiency 43 ofthe SCR catalyst 32 by summing values of the instantaneous efficiencyweighted by the first set of coefficients 48. As described above withrespect to the system 22, each instantaneous efficiency value may be aspecific data point or be determined over a specific sample size thatcan be defined by length of time or accumulated mass of NO_(x) and thenintegrated to arrive at a specific value.

If, on the other hand, the most recent value of the instantaneousefficiency of the SCR catalyst is equal to or below the instantaneousefficiency threshold 46, the method proceeds to frame 68. In frame 68,the method includes determining the overall operating efficiency 43 ofthe SCR catalyst 32 by summing values of the instantaneous efficiencyweighted by the second set of coefficients 50. As described relative toFIG. 1, the first and second sets of coefficients 48, 50 may be used aspart of the determination of the overall operating efficiency 43 of theSCR catalyst 32 in a EWMA filter programmed into the controller 42.Accordingly, the sum of the weighted instantaneous efficiency values ofthe SCR catalyst 32 generated by the EWMA filter is representative ofthe overall operating efficiency 43 of the SCR catalyst at anyparticular moment in time.

Following the determination of the overall operating efficiency 43 ofthe SCR catalyst 32 in frame 66 or 68, the method moves on to frame 70.In frame 70, the method includes determining whether the overalloperating efficiency 43 of the SCR catalyst 32 has dropped below anoverall operating efficiency threshold 52. From frame 70, the methodproceeds to frame 72. In frame 72, the method includes reporting whenthe overall operating efficiency 43 of the SCR catalyst 32 has droppedbelow the overall operating efficiency threshold 52. As described aboverelative to FIG. 1, a sensory signal 54 may be communicated to anddisplayed on the instrument panel 56 as a visual alert to affect suchreporting. According to the method, the overall operating efficiency ofthe SCR catalyst 32 being below the overall operating efficiencythreshold 52 may be indicative of the SCR catalyst having failed.

While the best modes for carrying out the invention have been describedin detail, those familiar with the art to which this invention relateswill recognize various alternative designs and embodiments forpracticing the invention within the scope of the appended claims.

1. A method of assessing overall operating efficiency of a selectivecatalytic reduction (SCR) catalyst, the method comprising: monitoringinstantaneous efficiency of the SCR catalyst; determining the overalloperating efficiency of the SCR catalyst by summing values of theinstantaneous efficiency weighted by a first set of coefficients if themost recent value of the instantaneous efficiency of the SCR catalyst isabove an instantaneous efficiency threshold; determining the overalloperating efficiency of the SCR catalyst by summing values of theinstantaneous efficiency weighted by a second set of coefficients if themost recent value of the instantaneous efficiency of the SCR catalyst isequal to or below the instantaneous efficiency threshold; determiningwhether the overall operating efficiency of the SCR catalyst has droppedbelow an overall operating efficiency threshold; and reporting when theoverall operating efficiency of the SCR catalyst has dropped below theoverall operating efficiency threshold; wherein the second set ofcoefficients weighs the most recent instantaneous efficiency value moreheavily than preceding instantaneous efficiency values as compared withthe first set of coefficients.
 2. The method of claim 1, wherein saiddetermining the overall operating efficiency of the SCR catalyst bysumming values of the instantaneous efficiency weighted by the secondset of coefficients is accomplished for a predetermined number of valuesof the instantaneous efficiency.
 3. The method of claim 2, wherein saiddetermining of the overall operating efficiency of the SCR catalystincludes employing an exponentially-weighted moving average (EWMA)filter to decrease the effect of more recent instantaneous efficiencyvalues on the overall operating efficiency determination.
 4. The methodof claim 2, wherein the SCR catalyst is employed for reducing oxides ofnitrogen (NO_(x)) emissions from an internal combustion engine.
 5. Themethod of claim 4, wherein the engine is a diesel type and the SCRcatalyst employs a diesel-exhaust-fluid (DEF) as a reductant suppliedfrom a reservoir for reducing NO_(x) emissions.
 6. The method of claim4, wherein the predetermined number of values of the instantaneousefficiency is collected during a single key cycle of the engine.
 7. Themethod of claim 1, wherein the overall operating efficiency of the SCRcatalyst being below the overall operating efficiency threshold isindicative of the SCR catalyst having failed.
 8. The method of claim 1,wherein each of said monitoring instantaneous efficiency of the SCRcatalyst, determining the overall operating efficiency of the SCRcatalyst, determining whether the overall operating efficiency of theSCR catalyst has dropped below the overall operating efficiencythreshold, and reporting when the overall operating efficiency of theSCR catalyst has dropped below the overall operating efficiencythreshold is accomplished by a controller.
 9. The method of claim 1,wherein said reporting when the overall operating efficiency of the SCRcatalyst has dropped below the overall operating efficiency threshold isaccomplished by generating an alert to an operator of the engine.
 10. Asystem for assessing overall operating efficiency of aselective-catalytic-reduction (SCR) catalyst employed for treatingexhaust emissions from an internal combustion engine, the systemcomprising: a passage configured to direct an exhaust gas stream fromthe engine to the SCR catalyst; a reservoir configured to supply areductant through the passage and into the SCR catalyst for reducingoxides of nitrogen (NO_(x)) in the exhaust gas stream; a controllerconfigured to: monitor instantaneous efficiency of the SCR catalyst;determine the overall operating efficiency of the SCR catalyst bysumming values of the instantaneous efficiency weighted by a first setof coefficients if the most recent value of the instantaneous efficiencyof the SCR catalyst is above an instantaneous efficiency threshold;determine the overall operating efficiency of the SCR catalyst bysumming values of the instantaneous efficiency weighted by a second setof coefficients if the most recent value of the instantaneous efficiencyof the SCR catalyst is equal to or below the instantaneous efficiencythreshold; determine whether the overall operating efficiency of the SCRcatalyst has dropped below an overall operating efficiency threshold;and report when the overall operating efficiency of the SCR catalyst hasdropped below the overall operating efficiency threshold; wherein thesecond set of coefficients weighs the most recent instantaneousefficiency value more heavily than preceding instantaneous efficiencyvalues as compared with the first set of coefficients.
 11. The system ofclaim 10, wherein the controller determines the overall operatingefficiency of the SCR catalyst by summing values of the instantaneousefficiency weighted by the second set of coefficients for apredetermined number of values.
 12. The system of claim 11, wherein thecontroller determines the overall operating efficiency of the SCRcatalyst by employing an exponentially-weighted moving average (EWMA)filter to decrease the effect of more recent instantaneous efficiencyvalues on the overall operating efficiency determination.
 13. The systemof claim 10, wherein the controller is configured to report when theoverall operating efficiency of the SCR catalyst has dropped below theoverall operating efficiency threshold by generating an alert to anoperator of the engine.
 14. The system of claim 10, wherein the engineis a diesel type and the reductant is a diesel-exhaust-fluid (DEF). 15.The system of claim 10, wherein the predetermined number of values ofthe instantaneous efficiency is collected during a single key cycle ofthe engine.
 16. The system of claim 10, wherein the overall operatingefficiency of the SCR catalyst being below the overall operatingefficiency threshold is indicative of the SCR catalyst having failed.17. A vehicle comprising: an internal combustion engine configured topropel the vehicle; an exhaust passage configured to direct an exhaustgas stream from the engine to a selective-catalytic-reduction (SCR)catalyst; a reservoir configured to supply a reductant through thepassage and into the SCR catalyst for reducing oxides of nitrogen(NO_(x)) in the exhaust gas stream; and a controller configured to:monitor instantaneous efficiency of the SCR catalyst; determine anoverall operating efficiency of the SCR catalyst by summing values ofthe instantaneous efficiency weighted by a first set of coefficients ifthe most recent value of the instantaneous efficiency of the SCRcatalyst is above an instantaneous efficiency threshold; determine theoverall operating efficiency of the SCR catalyst by summing values ofthe instantaneous efficiency weighted by a second set of coefficients ifthe most recent value of the instantaneous efficiency of the SCRcatalyst is equal to or below the instantaneous efficiency threshold;determine whether the overall operating efficiency of the SCR catalysthas dropped below an overall operating efficiency threshold; and reportwhen the overall operating efficiency of the SCR catalyst has droppedbelow the overall operating efficiency threshold; wherein the second setof coefficients weighs the most recent instantaneous efficiency valuemore heavily than preceding instantaneous efficiency values as comparedwith the first set of coefficients; and wherein the overall operatingefficiency of the SCR catalyst being below the overall operatingefficiency threshold is indicative of the SCR catalyst having failed.18. The vehicle of claim 17, wherein the controller determines theoverall operating efficiency of the SCR catalyst by summing values ofthe instantaneous efficiency weighted by the second set of coefficientsfor a predetermined number of values during a single key cycle of theengine.
 19. The vehicle of claim 18, wherein the controller determinesthe overall operating efficiency of the SCR catalyst by employing anexponentially-weighted moving average (EWMA) filter to decrease theeffect of more recent instantaneous efficiency values on the overalloperating efficiency determination.
 20. The vehicle of claim 17, whereinthe engine is a diesel type and the reductant is a diesel-exhaust-fluid(DEF).